The EPA Environmental Monitoring and Assessment Program (United States Environmental Protection Agency, 1991) distinguishes environmental indicators on the basis of whether they best measure stresses, exposures, or responses. Stressor indicators identify activities that have an impact on the environment. These include land-use changes and discharges of pollutants. Exposure indicators identify components of the environment that have been subjected to a substance or activity that could potentially change the environment directly or indirectly. Response indicators detect the status of a particular resource component, usually biological, in relation to external stresses and exposure to those stresses. Particular indicators function most appropriately in one of the three indicator categories, although they may double as a secondary evaluator of another indicator class. For example, chemical measures generally function best as exposure indicators but may indirectly provide insights to response. Biological measures are inherently response oriented and may or may not provide more than qualitative insights to exposure and stressors.
These comparisons of chemical and biological indicators illustrate a national problem: the inappropriate use of stressor and exposure indicators as substitutes for response indicators. States that do not have well-developed bioassessment programs still must report on the status of their waters to EPA on a biennial basis. Thus, they are forced to use whatever information is available. Usually, the readily available information is in the form of stressor or exposure indicators. In attempting to resolve the obvious inconsistencies in measuring the condition of aquatic resources, a fundamental step is to recognize and establish appropriate roles for the different chemical, physical, and biological indicators. An accurate portrayal of the condition of the nation's surface waters depends on the use of suites of these indicators, each in their appropriate role as stressor, exposure, or response indicators.
Without a sound theoretical basis, it would be difficult if not impossible to develop biological criteria and meaningful measures of ecological condition. Obvious ecological degradation such as fish kills stimulated the landmark environmental legislation of the past 2 decades, but that biological focus was lost in the quest for easily measurable water-quality indicators (Karr, 1991). The biological integrity provision of the Clean Water Act, which was initially difficult to specify in practice (Ballentine and Guarria, 1975), was eventually defined by Karr and Dudley ( 1981 ) as ". . . the ability of an aquatic ecosystem to support and maintain a balanced, integrated, adaptive community of organisms having a spe-